In the prior art, resistance spot welds are made by passing current through the workpieces by electrodes which apply the welding force. When welding a highly conductive metal such as aluminum, high current must be used to create enough heat to fuse the workpieces. Most of the heat is generated at the faying surface between the two workpieces, and at the work surface between the workpiece and the electrode tip.
In general, it is desirable to generate as little heat as possible at the work surface so as to avoid impairing workpiece surface quality or shortening the electrode tip life. Preferably, most of the heat is generated at the faying surface to weld the workpieces together.
When spot welding aluminum workpieces such as mill surface aluminum sheet, oxides present on the workpiece surfaces interfere with the electrical contact between the electrode tip and workpiece surface. Consequently, excessive heat is generated at the work surface rather than at the faying surface due to the increased electrical resistance caused by the presence of the surface oxides.
This excessive heat generation at the work surface causes sticking of the electrode tip to the surface and pick up of aluminum onto the tip. As result of these effects, the surface quality of the workpieces is impaired and the electrode tip life is shortened.
As such, a need has developed to provide improved apparatus and methods for spot welding workpieces, especially aluminum sheet, which concentrates the welding heat at the faying surface rather than at the workpiece surface. There is also a need to reduce the pick up of aluminum on the electrode tip and increase the electrode tip life.
In response to this need, the present invention provides a spot welding method and apparatus which utilizes a vibrating or twisting motion of the spot welding head to effectively break up any surface oxide present on the workpiece to be welded. This "breaking up" avoids excessive heat generation at the workpiece surface, thereby concentrating the heat at the faying surface for improved spot welding.
In the prior art, apparatus that oscillate electrodes for arc welding are known. U.S. Pat. No. 4,704,513 to Sugitani et al. discloses a groove tracing control method for a fillet welding which rotates a welding torch and thereby rotates an arc thereof at high speed. As shown in FIG. 9 of this patent, the filler wire is rotated thereby producing a weld bead having uniform leg lengths.
U.S. Pat. No. 4,047,656 to McCombs discloses a welding head for metallic arc inert gas welding which is moved by an external guidance system over a workpiece. The external guidance system moves a spindle in a circular or orbital path over the work surface.
In these patents, the oscillating or orbital movement of the filler material is used in combination with longitudinal movement of the welding head to fill the spaces between the workpieces to be welded with filler metal. There is no recognition in these patents of the problems caused by surface oxides in aluminum sheet spot welding.